In a capillary tube, how do we tell which side of the meniscus has higher or lower pressure

A liquid with a sharp contact angle (e.g., water on glass) will form a concave meniscus, and the liquid pressure under the meniscus will be smaller than the atmospheric pressure

Explanation:

The phenomenon of capillarity is produced by the action of the surface tension of the fluids and is observed when a small diameter tube is immersed within the fluid. If we pay attention to the result, we can see that, depending on the fluid, two different things can happen, that the liquid rises through the tube and that the level inside the tube is greater than that of the liquid or that the opposite happens.

The case in which the liquid rises above the tube occurs when the liquid "wets". This occurs when the adhesion forces with the walls exceed those of cohesion between the fluid molecules. In this case, the concave side is out of the fluid.

The case where the level of the liquid inside the tube is lower than the level of the liquid occurs when the liquid does not get wet. We remember that the liquid does not get wet when the cohesion forces are greater than those of adhesion. This phenomenon is called capillary depression and the concave angle is for the liquid side and is said to be convex.

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.